HgCdTe-based bias-switchable dual-band (two-color) infrared detectors have been reported in the literature since the early 1990s. The typical device structure consists of two back-to-back infrared photodiodes, each designed to detect a particular color band. The two diodes are monolithically integrated, typically in an n-P-N (capital letters denote material layers with wider band gap) three-layer design or an n-p-P-N four-layer design. (See, e.g., J. M. Arias, et al., J. Appl. Phys. 70, 4620-4622 (1991); E. R. Blazejewski, et al., J. Vac. Sci. Technol. B10, 1626-1632 (1992); and M. B. Reine, et al., J. Electron. Mater. 24, 669-679 (1995), the disclosures of each of which are incorporated herein by reference. The topic has been well documented in books and review articles. (See, e.g., A. Rogalski, Infrared Physics & Technology 41, 231-238 (2000), the disclosure of which is incorporated herein by reference.)
Recently, type-II superlattices (e.g., InAs/GaSb or InAs/GaInSb) have emerged as viable alternatives to HgCdTe for infrared detection. The performance of type-II superlattice based infrared detectors can be enhanced by using heterostructure designs such as the nBn, pBp, double heterostructure (DH), or complementary barrier infrared detector (CBIRD). (See, e.g., A. M. White, U.S. Pat. No. 4,679,063; S. Maimon and G. W. Wicks, Appl. Phys. Lett. 89(15) 151109 (2006); S. Maimon, U.S. Pat. No. 7,687,871 B2; J. L. Johnson, et al., Appl. Phys. Lett. 80(2) 1116-1127 (1996); B.-M. Nguyen, et al., Appl. Phys. Lett. 93(12) 123502 (2008); and D. Z.-Y. Ting, et al., Appl. Phys. Lett. 95, 023508 (2009), the disclosures of each of which are incorporated herein by reference.) These device structures make use of unipolar barriers, which can block one carrier type (electron or hole) but allow the un-impeded flow of the other. Type-II superlattices have also been used in bias-switchable dual-band infrared detectors. A simple method involves the two-color nBn design, where an electron-blocking (but not hole-blocking) unipolar barrier is inserted between two n-doped infrared absorbers with different energy band gaps. Dual-band nBn detectors implemented using type-II InAs/Ga(In)Sb superlattices have been reported in the literature. (See, A. Khoshakhlagh, et al., Appl. Phys. Lett. 91, 263504 (2007), the disclosure of which is incorporated herein by reference.)
A general concern with using n-doped type-II superlattices as infrared absorbers has to do with the unfavorable hole mobility characteristics. Semiconductor transport is controlled primarily by the properties of minority carriers, which are holes in the case of n-type material. The hole mobility of a type-II superlattice such as InAs/GaSb is generally highly anisotropic. Hole mobility is considerably lower in the direction perpendicular to the superlattice layer interfaces than in the lateral (in-plane) directions. Theoretical considerations show that this effect is more pronounced in long wavelength infrared (LWIR) superlattices than mid-wavelength infrared (MWIR) superlattices. (See, David Z. Ting, et al., Proc. of SPIE 7419, 74190B (2009), the disclosure of which is incorporated herein by reference.) Strong lateral diffusion of minority carriers in superlattice nBn photodetector structures has been observed experimentally. (See, E. Plis, et al., Appl. Phys. Lett. 93, 123507 (2008), the disclosure of which is incorporated herein by reference.) The fact that holes have more difficulty diffusing along the perpendicular direction toward the collecting contact than diffusing laterally can be very problematic in a focal plane array (FPA). For an FPA with reticulated pixels (physically isolated pixels, defined by etching), lateral diffusion transports the minority carriers to the exposed pixel sidewalls, where recombination could take place readily. In a planar-processed FPA with non-reticulated pixels, strong lateral diffusion means that minority carriers can spread easily to neighboring pixels, resulting in image blurring.
An alternative approach is to use a dual-band pBp design instead. A pBp detector structures consists of a hole-blocking (but not electron blocking) unipolar barrier sandwiched between two p-doped infrared absorbing superlattices with different energy band gaps. In type-II superlattices such as InAs/GaSb, electron mobility is high and nearly isotropic. The problems associated with unfavorable anisotropic hole mobilities in n-type superlattice infrared absorbers are greatly reduced in pBp structures where electrons are minority carriers. However, the pBp structure has its own problems. Unlike the nBn design which is capable of reducing electron surface leakage Current (See, G. W. Wicks, G. R. Savich, J. R. Pedrazzini, and S. Maimon, “Infrared detector epitaxial designs for suppression of surface leakage current,” Proc. of SPIE 7608, 760822 (2010), the disclosure of which is incorporated herein by reference.), the pBp design is susceptible to this mechanism. Consider a reticulated detector pixel with exposed sidewalls. A type-II superlattice containing InAs layers is likely to develop an accumulation of electrons on the sidewall surface (InAs surface Fermi level is pinned in the conduction band, independent of doping type.). This turns the sidewall surface into n-type. Since there are no electron barriers in a pBp structure, electron surface leakage current flow un-impeded from one electrode to the other. This can result in a sizable dark current that reduces detector sensitivity.
Accordingly, a need exists for a practical bias-switchable dual-band infrared detector.